High extracellular levels of glutamate have been recognized as a significant biochemical consequence of various forms of neuronal injury, including ischemic, degenerative and traumatic brain damage. Further, it has been proposed that the elevated glutamate levels exacerbate the primary insult, possibly by acting at excitatory glutamate receptors. While it was previously believed that the source of the excess glutamate was neuronal stores, it is now recognized that at least some of the glutamate may be the result of enzymatic conversion of glutamine to glutamate by glutaminase.
U.S. Pat. No. 5,158,976 (Rosenberg) discloses the general idea of blocking glutaminase for neuroprotection, but does not appreciate the advantages of using membrane impermeant compounds, as described herein. The approach described by Rosenberg is directed to inhibiting intracellular glutamine metabolism and production of glutamate and does not appreciate that most of the extra glutamate in damaged neuronal tissue is produced by a glutaminase activity that is exposed to the extracellular milieu, presumably as a result of cell damage, as described herein. While Rosenberg describes the systemic toxicity that can ensue when glutaminase is inhibited systemically, it does not appreciate that such toxicity can be reduced considerably by selecting a glutaminase inhibitor that is impermeant to cell membranes and, preferably, specific for the mitochondrial form of the enzyme present in neuronal cells.
According to the discoveries underlying the present invention, membrane impermeant inhibitors having certain characteristics defined herein are both neuroprotective and selectively inhibit glutamate production by damaged neurons. Glutaminase in intact neurons, as well as glutamatergic transmission, are unaffected by such inhibitors, reducing systemic side effects and toxicity.